Soft magnetic materials can be used for applications such as core materials in inductors, stators, and rotors for electrical machines, actuators, sensors, and transformer cores. Traditionally, soft magnetic cores, such as rotors and stators in electric machines, are made of stacked steel-sheet laminates. However, in the last few years there has been a keen interest in so called Soft Magnetic Composite (SMC) materials. The SMC materials are based on soft magnetic particles, usually iron based, with an electrically insulating coating on each particle. By compacting the insulated particles, optionally together with lubricants and/or binders, using the traditionally powder metallurgy process, the SMC parts are obtained. By using the powder metallurgical technique it is possible to produce materials having a higher degree of freedom in the design of the SMC part compared to using steel-sheet laminates, as the SMC material can carry a three dimensional magnetic flux and as three dimensional shapes can be obtained with the compaction process.
As a consequence of the increased interest in the SMC materials, improvements of the soft magnetic characteristics of the SMC materials is the subject of intense studies in order to expand the utilisation of these materials.
In order to achieve such improvement, new powders and processes are continuously being developed.
Two key characteristics of an iron core component are its magnetic permeability and core loss characteristics. The magnetic permeability of a material is an indication of its ability to become magnetised or its ability to carry a magnetic flux. Permeability is defined as the ratio of the induced magnetic flux to the magnetising force or field intensity. When a magnetic material is exposed to an alternating field, such as for example an alternating electric field, energy losses occur due to both hysteresis losses and eddy current losses. The hysteresis loss is brought about by the necessary expenditure of energy to overcome retained magnetic forces within the iron core component and is proportional to the frequency of e.g. the alternating electrical field. The eddy current loss is brought about by the production of electric currents in the iron core component due to the changing flux caused by alternating current (AC) conditions and is proportional to the square of the frequency of the alternating electrical field. A high electrical resistivity is then desirable in order to minimise the eddy currents and is of special importance at higher frequencies, such as for example above about 60 Hz. In order to decrease the hysteresis losses and to increase the magnetic permeablity of a core component it is generally desired to heat-treat a compacted part whereby the induced stresses from the compaction are reduced. Furthermore, in order to reach desired magnetic properties, such as high magnetic permeability, high induction and low core losses, high density of the compacted part is often needed. High density is here defined as a density above 7.0, preferably above 7.3 most preferably about 7.5 g/cm3 for an iron-based compacted part.
In addition to the soft magnetic properties, sufficient mechanical properties are essential. High mechanical strength is often a prerequisite to avoid introducing cracks, laminating, and break-outs and to achieve good magnetic properties of compacts which after compaction and heat treatment have been subjected to machining operations. Also, lubricating properties of an impregnated polymer network can increase the lifetime of cutting tools considerably.
In order to be able to expand the utilisation of SMC components, high strength at elevated temperature is an important property such as for example for components used in applications such as motor cores, ignition coils, and injection valves in automobiles.
By admixing a binder to the SMC powder before compaction, improved mechanical strength of the compacted and heat treated component can be obtained. In the patent literature several kinds of organic resins, such as thermoplastics and thermoset resins, inorganic binders such as silicates or silicon resins, are reported. The heat treatment of organic resin bonded components is restricted to comparatively low temperatures, below about 250° C., as the organic material destroys at temperature above about 250° C. The mechanical strength of heat treated organic bonded components at ambient conditions is good, but deteriorates above 100° C. Inorganic resins can be subjected to higher temperatures without effecting the mechanical properties, however, the use of inorganic binders are often associated with poor powder properties, poor compressibility, poor machinability and often needed in high amounts that precludes higher density levels.
U.S. Pat. No. 6,485,579 describes a method of increasing the mechanical strength of SMC component by heat treating the component in the presence of water vapour. Higher values for the mechanical strength are reported compared to components heat treated in air, however, increased core losses are obtained. A similar method is described in WO2006/135324 where high mechanical strength in combination with improved magnetic permeability are obtained provided metal free lubricants are used. The lubricants are evaporated in a non-reducing atmosphere before subjecting the component to water vapour. However, the oxidation of the iron particles, when the component is subjected to steam treatment, will also increase the coercive forces and thus core losses.
Impregnation, infiltration, and sealing of die casts or powder metal (P/M)-components, e.g. by an organic network, are known methods in order to prevent surface corrosion or seal surface porosity. Highly dependent on density and processing conditions of P/M parts, the degree of penetration of the organic network will vary. Low density levels (<89% of the theoretical density) and mild sintering conditions or heat treatments provide for easy penetration and full impregnation. For high performance materials having high density and low porosity the prerequisites to reach full impregnation are limited.
Impregnation of SMC components to improve the machinability for producing prototype components, or to improve the corrosion resistance, is shown for example in patent application JP 2004 178 643 where the impregnation liquid constitutes of oils in general. Besides the marginally improved machinability of this method it results in greasy and slippery surfaces, worse to handle. Oil does not greatly improve cutting tool life because it never becomes solid. In the same way, uncured or soft sealants offer little value to machining. A reliable cure mechanism for the polymer together with high mechanical strength of the composite part is the best assurance of consistent machining performance.
U.S. Pat. Nos. 6,331,270 and 6,548,012 both describe processes for manufacturing AC soft magnetic components from non-coated ferromagnetic powders by compaction of the powders together with a suitable lubricant followed by heat treatment. It is also stated that for applications requiring higher mechanical strength, the components may be impregnated, for example with epoxy resin. As non-coated powders are used, these methods are less suitable due to high eddy current losses obtained if the components are used for applications subjected to higher frequencies, above about 60 Hz. U.S. Pat. No. 5,993,729 deals mainly with uncoated iron-based powder and infiltration of low density compacts produced with the aid of die wall lubrication. The patent also mentions powders, wherein the particles are individually coated with a non-binding electro-insulating layer, comprising of oxides applied either by sol-gel process or by phosphatation. The compacted soft magnetic elements according to U.S. Pat. No. 5,993,729, are restricted to applications working at low frequencies, below about 60 Hz, due to poor electrical resistivity. In addition, the oxidative heat treatment of powder or compacts before the impregnation process will restrict or fully prevent pore penetration of the impregnating liquid, especially for compacts of high density, above about 7.0 g/cm3, and especially above about 7.3 g/cm3.